Immunohistochemical studies revealed a wide distribution of pancreatic polypeptide (PP) immunoreactive cell bodies and nerve fibers in the rat central and peripheral nervous systems. A number of PP-immunoreactive cells were demonstrated to coexist with catecholamine neurons in the pons-medulla and sympathetic ganglia. Within the superior cervical ganglia, PP was found to coexist with norepinephrine in a subpopulation of the catecholamine neurons. This subpopulation of neurons was restricted to those innervating the blood vessels of the head and neck. Following electrical stimulation of the cervical sympathetic trunk, PP was coreleased with norepinephrine from the nerves. An immunohistochemical study of PP neurons in the monkey brain revealed a widespread innervation similar to that in the rat. Corticotropin releasing factor (CRF) was revealed in the rat CNS using an antibody to synthetic ovine CRF as an immunohistochemical marker. CRF immunoreactivity was observed in the hypothalamus, thalamus, amygdala, cerebral cortex, midbrain, pons, medulla and spinal cord. CRF was not observed in he peripheral nervous system. Immunohistochemicl studies of Gamma3-MSH revealed a distribution of immunoreactive neurons limited to the arcuate nucleus and basomedial hypothalamus. These cells were demonstrated to contain other neuropeptides (Alpha-MSH, Beta-endorphin, ACTH) also derived from pro-opiomelanocortin.